(I will abbreviate schizophrenia to "sz" in the following).  

I have EF Torrey's book "surviving schizophrenia".  On p365 there's a
chart, "what are my chances of getting sz?"

It lists the odds with having 1 grandparent w/ sz - about 4 percent. 

What I need to know is the odds if TWO grandparents have sz (without
any other family member having it).  For example, what if my grandma
on my mother's side has it, and also my grandmother on my father's
side.  What's the best estimate then?

Try a search for The Myth of mental illness